1. Field of the Invention
The present invention relates to a liquid-ejecting recording head including nozzle arrays that apply recording liquid onto a recording medium, and to a liquid-ejecting recording apparatus in which the liquid-ejecting recording head is mounted. The present invention can be applied to a liquid-ejecting recording head including nozzle arrays that apply recording liquid onto a recording medium, and to a liquid-ejecting recording apparatus in which the liquid-ejecting recording head is mounted. Further, the present invention can be applied to a liquid-ejecting recording head including nozzle arrays that are scanned in two directions so that a recording liquid of a specific color is applied in a symmetrical order with respect to a recording liquid of another color, and to a liquid-ejecting recording apparatus in which the liquid-ejecting recording head is mounted.
2. Description of the Related Art
Image forming apparatuses record images on a recording medium, such as a paper sheet or a thin plastic plate, according to image information (including character information). Image forming apparatuses are classified, for example, into a liquid ejection type, a wire dot type, a thermal type, and a laser beam type by recording method. Among these types, the liquid-ejection type image forming apparatus performs recording by discharging droplets of recording liquid from a liquid-ejecting recording head (hereinafter sometimes abbreviated as a recording head) onto a recording medium. In the liquid-ejection type image forming apparatus, size reduction of the recording means is easy, high-speed recording of high-definition images is possible, and the operating cost is low. Since recording is performed in a non-impact manner, the noise level is low. In addition, a color image can be easily recorded with recording liquids of multiple colors.
In general, bubbles are formed in a recording-liquid flow passage, a common liquid chamber, and a discharging-energy generating chamber provided in a recording head. Bubbles are formed by printing or suction recovery, or formed when the recording head is left unused for a long time. These bubbles may form a lump of bubble, which can hinder the supply of recording liquid and induce a non-discharging phenomenon of recording liquid. The lump of bubble in the recording head is sucked and removed by a suction recovery unit provided in the main body of the liquid-ejecting recording apparatus.
The suction recovery unit instantaneously generates a high negative pressure, separates bubbles from walls of a recording-liquid flow passage, a common liquid chamber, and a discharging-energy generating chamber, and applies suction to remove the bubbles together with the recording liquid. However, if suction is performed with a low negative pressure of suction, bubbles are not sufficiently removed from the recording head.
The low negative pressure of suction refers to a negative pressure such as not to draw in bubbles from a recording-liquid storage tank that includes an absorber for holding recording liquid. In contrast, when suction is performed with a high negative pressure of suction, the balance between the recording-liquid holding force (capillary force) of the absorber and the negative pressure of suction is disrupted. As a result, bubbles are drawn in together with the recording liquid from the recording-liquid storage tank. These new bubbles drawn in the recording head induce a non-discharging phenomenon of recording liquid.
In this way, it is important to control the negative pressure in the suction recovery operation. This suction recovery operation allows the liquid-ejecting recording apparatus to always stably discharge the recording liquid.
Suction recovery using the suction recovery unit is performed while all nozzle arrays are covered with a cap or while a plurality of groups of nozzle arrays are covered with the respective caps. In general, a plurality of nozzle arrays provided in the same member are capped collectively.
In any case, a suction may be applied to the recording head while a cap formed of an elastic material, such as rubber, is in tight contact with the peripheries of the nozzles. As a result, different kinds of recording liquids may be removed simultaneously from the recording head by applying suction thereto. Further, suction recovery is performed by reducing the pressure in the cap by a suction pump.
Unfortunately, if the relative difference in flow resistance (flow resistance ratio) among the recording-liquid flow passages increases, the balance of suction amount among the recording-liquid flow passages is disrupted, and it is difficult to collectively subject all the nozzles to suction recovery. Examples of recording heads in which the flow resistance ratio of the recording-liquid flow passages is high will be given below.
(1) A recording head having a nozzle layout in which nozzle arrays corresponding to specific recording liquids are symmetrically arranged with respect to a nozzle array corresponding to another recording liquid (hereinafter referred to as “symmetrically arranged nozzle arrays”). In general, cyan and magenta recording liquids are defined as the specific recording liquids. A recording-liquid flow passage communicating with each of the symmetrically arranged nozzle arrays is bifurcated. That is, the number of nozzles included in each of the symmetrically arranged nozzle arrays is about double the number of nozzles included in another nozzle array that is not symmetrically arranged (hereinafter referred to as a “single nozzle array”). For this reason, the flow resistance in the recording-liquid flow passages communicating with the symmetrically arranged nozzle arrays is lower than the flow resistance in the recording-liquid flow passage communicating with the single nozzle array. Therefore, the amount of recording liquid to be sucked from each of the symmetrically arranged nozzle arrays is larger than that of the single nozzle array.
(2) A recording head in which a nozzle array corresponding to a specific recording liquid includes nozzles having a large diameter (large nozzles) and nozzles having a small diameter (small nozzles) (hereinafter refereed to as a “large and small nozzle array”), and a nozzle array corresponding to another recording liquid includes only large nozzles (hereinafter referred to as a “large nozzle array”). However, the total number of large and small nozzle arrays is substantially equal to the total number of large nozzle arrays. For this reason, the flow resistance in a recording-liquid flow passage communicating with the large nozzle array is lower than the flow resistance in a recording-liquid flow passage communicating with the large and small nozzle array. Therefore, the amount of recording liquid to be sucked from the large nozzle array is larger than that of the large and small nozzle array.
(3) A recording head in which symmetrically arranged nozzle arrays each include large nozzles and small nozzles, and a single nozzle array includes only large nozzles. In this case, the flow resistance in recording-liquid flow passages communicating with the symmetrically arranged nozzle arrays is higher than the flow resistance in a recording-liquid flow passage communicating with the single nozzle array. However, the difference in flow resistance between the nozzle arrays is smaller than in the above-described recording head (1).
(4) A recording head including a nozzle array having only small nozzles and a nozzle array having only large nozzles. In this case, the flow resistance in a recording-liquid flow passage communicating with the nozzle array having only large nozzles is lower than the flow resistance in a recording-liquid flow passage communicating with the nozzle array having only small nozzles.
In recent liquid-ejecting recording apparatuses, enhancement of recording quality and increase of the recording speed in color printing are important themes. A high-quality image can be recorded in many gradation levels by discharging recording-liquid droplets onto a recording medium so as to form dots having different areas.
In a typical recording head, two nozzle arrays extend in parallel and in a direction orthogonal to a head scanning direction. In normal cases, one of the nozzle arrays is a large nozzle array, and the other nozzle array is a small nozzle array. The large nozzle array and the small nozzle array communicate with a common recording-liquid supply port so that the same kind of recording liquid is supplied to the nozzle arrays. That is, recording-liquid droplets are discharged onto the recording medium to form dots having different areas by dot modulation that allows large droplets and small droplets to be discharged selectively.
FIG. 14A is a schematic plan view showing the surroundings of a liquid-ejecting section in a recording head having the above-described configuration as the related art. A plurality of nozzle passages 403 and a plurality of energy generating elements 404 are provided on a substrate 402. Nozzles 411 and 412 are provided so as to oppose the energy generating elements 404. An orifice plate 410 is joined to the substrate 402, as shown in FIG. 14B serving as a cross-sectional view taken along line XIVB-XIVB in FIG. 14A.
The nozzles 411 form an array, and the nozzles 412 form another array. The diameter of the nozzles 411 is different from that of the nozzles 412. Correspondingly, the area of regions in which the energy generating elements 404 apply energy to the recording liquid is different between the arrays of the nozzles 411 and 412. The width of the nozzle passages 403 is also different between the arrays of the nozzles 411 and 412. More specifically, the diameter of the nozzles, the areas of the energy generating elements 404, and the width of the nozzle passages 404 in the right array of the nozzles 411 are larger than in the left array of the nozzles 412. Therefore, the volume of droplets of recording liquid discharged from the nozzles 411 in the left array is smaller than the volume of droplets of recording liquid discharged from the nozzles 412 in the right array. As a result, it is possible to discharge two kinds of recording-liquid droplets having different volumes.
Accordingly, recording can be performed in more gradation levels than when the recording head discharges only recording-liquid droplets having a large volume. Moreover, recording can be performed at a higher speed than when the recording head discharges only recording-liquid droplets having a small volume. Since the ratio of the large droplets and the small droplets can be freely determined, one recording head can have a wide range of recording characteristics.
In order to maintain high image quality, it is necessary to prevent the entry of foreign substances that adversely affect discharging by the recording head. This is because print quality is reduced when the nozzles and flow passages are clogged with foreign substances or dust. Accordingly, a porous member (filter) is provided in a recording-liquid introducing portion of a typical recording head. The filter needs to trap foreign substances and dust smaller than the diameter of nozzles and the size of flow passages. That is, the required trap ability of the porous member is determined by the diameter of nozzles and the size of flow passages. The ability is generally expressed as mesh roughness.
The flow resistance of the entire recording head is substantially determined by the pressure losses in the nozzles 411 and 412, the nozzle passages 403, and the filter. That is, enhancing the trapping ability of the filter means increasing the flow resistance of the entire recording-liquid supply passage.
FIG. 15 is an external perspective view of the recording head as the related art. FIGS. 16A to 16C are partial cross-sectional views showing the cross-sectional shape of a recording-liquid introducing portion in the recording head. FIG. 16A shows a state before the filter is fixed, FIG. 16B shows a state immediately after the filter is welded, and FIG. 16C shows a state in which the filter is in contact with a press contact member.
Referring to FIG. 15, the recording head includes recording element substrates 200, a first plate 201 serving as a support substrate, a sheet electric wiring board 202, a contact-terminal wiring board 203, a second plate 204, a flow passage forming member 205, and screws 206.
As shown in FIGS. 16A to 16C, a filter 207 is fixed to a recording-liquid introducing portion 210 provided in the flow passage forming member 205 by heat welding. The recording-liquid introducing portion 210 includes a welding rib 211 that welds the filter 207, and a covering rib 212 that covers the edge of the filter 207 from the periphery. These ribs 211 and 212 fix the filter 207. A plurality of columns 213 stand at the back of the filter 207 so as to support the filter 207. A press contact member 220 is provided in a recording-liquid supply portion of a recording-liquid storage tank (not shown). By contact between the filter 207 and the press contact member 220, recording liquid is supplied (FIG. 16C). In this case, it is necessary to reliably attach the filter 207 so that a fiber edge around the filter 207 does not scratch the press contact member 220 and the filter 207 does not separate. For that purpose, the edge of the filter 207 is prevented from being exposed by being covered with resin of the flow passage forming member 205 (see U.S. Pat. No. 6,592,215).
In order to ensure proper contact between the filter 207 and the press contact member 220, it is necessary to appropriately set the shape of the contact portion, the relative positional accuracy, and the contact pressure. For purposes of proper contact and stable production, it is effective to form the filter 207 in the shape of a perfect circle.
The recording-liquid introducing portion 210 and the filter 207 are provided for each of the mounted recording-liquid storage tanks. While the filter 207 has a small diameter of several millimeters, the material cost thereof is high. Accordingly, lower production cost and higher productivity are achieved by using the same type of filters.
The following methods are effective in increasing the recording speed of the liquid-ejecting recording apparatus: (1) The length of the nozzle arrays in the recording head is increased. (2) The discharging (driving) frequency of the recording head is increased. (3) The number of printing passes is reduced, for example, by bidirectional printing.
In bidirectional printing, the energy needed to obtain the same throughput is dispersed in time, when compared with unidirectional printing. Therefore, bidirectional printing is markedly effective in terms of the cost of the total system.
Unfortunately, in bidirectional printing, the landing order of color ink droplets differs between the forward scanning direction and the backward scanning direction of the recording head. This causes a principle problem of band-shaped color unevenness. Since this problem results from the landing order of the recording liquid droplets, it appears more or less as a difference in color development when different color dots overlap with each other.
More specifically, recording liquid discharged earlier first dyes the recording medium from the surface to the inside so as to form a dot thereon. The subsequent recording liquid forms a dot so that the dot overlaps with the dot formed by the preceding recording liquid. Then, much recording liquid dyes a portion under the portion dyed by the preceding recording liquid. Therefore, the color of the preceding recording liquid tends to develops more. For this reason, when discharging nozzle arrays corresponding to different colors are arranged in order in the main scanning direction, band-shaped color unevenness occurs. That is, in bidirectional printing, the landing order of droplets of recording liquid is reversed between the forward scanning direction and the sub-scanning direction. As a result, band-shaped color unevenness is caused by the difference in color development.
Accordingly, there has been adopted a recording head in which recording liquids of specific colors (for example, cyan and magenta) are discharged in a symmetrical order with respect to another color. By adopting this recording head, cyan and magenta recording liquids can be discharged in the same order during bidirectional printing.
In recent developments of the recording head for higher recording speed and higher recording quality, not only the length of the nozzle arrays has been increased, but also nozzles having different diameters have been provided so as to correspond to the colors of recording liquids. In this recording head, a high trapping ability is required to the porous member. For that purpose, a fine-mesh porous member is adopted. However, this increases the flow resistance in the entire recording-liquid flow passage, and also increases the difficulty in controlling suction recovery in the apparatus body. Moreover, the increase in mesh density increases the cost of the porous member.
On the other hand, the relative difference in flow resistance among the recording-liquid flow passages increases, and the balance of the suction amount is disturbed. Consequently, it is difficult to simultaneously apply suction to a plurality of nozzle arrays by covering the nozzle arrays with a single cap.
More specifically, when a suction is applied to the nozzle arrays simultaneously, the suction amount from the recording-liquid flow passage having a low flow resistance is larger than the suction amount from the recording-liquid flow passage having a high flow resistance. In this way, when suction is conducted, with a great negative pressure, on the head in which the flow resistance is out of balance among the recording-liquid flow passages, the number of bubbles in the head increases. That is, in the flow passage having a low flow resistance, the supply amount of the recording-liquid storage tank increases, and bubbles are drawn into the flow passage together with the recording liquid. In contrast, in the flow passage having a high flow resistance, bubbles remaining in the flow passage are not sufficiently ejected because of a shortage of negative pressure of suction. The above-described problems become more remarkable as the relative difference in flow resistance between the recording-liquid flow passages increases. As a result, the number of bubbles remaining in the recording head increase.
With the increase of the relative difference in flow resistance between the recording-liquid flow passages, an amount of ink (recording liquid) wasted during a suction recovery process may be increased, thereby increasing an operating cost associated therewith.